This invention relates to fiber yarn twisting apparatus for preparing curled yarns, and more particularly to friction type yarn false twisting apparatus for applying false twists to a continuous synthetic yarn by contacting the yarn against the periphery of one or more rotating discs.
As shown in FIGS. 1 and 2, a prior art yarn false twisting apparatus comprises a first pair of friction discs 1, 3 and 5, and a second pair of friction discs 2, 4 and 6 which are mounted on spaced parallel shafts 7 and 8 respectively, the discs of the first and second pairs interleaving with each other. A yarn Y to be twisted is threaded such that it firstly passes through the interactions of the peripheries of the discs 1, 2 and 3 on one side of the assembly and then through the intersections of the peripheries of the discs 4, 5 and 6 on the opposite side. The discs are rotated in the direction of arrows so that the yarn is twisted several times by the friction between it and the discs. For this reason, at least the periphery of each disc is made of or covered by highly frictional material.
Although the friction type yarn false twisting apparatus described above is advantageous in that its construction is simple and the threading operation of the yarn is also easy, as the number of twists increases, the twisting of the yarn becomes unstable and nonuniform. Furthermore, twisting back or untwisting is not performed satisfactory due to stick slip phenomena of the yarn on the discs. Such unstable conditions becomes remarkable as the speed of the yarn increases. Although unstable conditions can be removed by increasing the tension of the yarn, fluffing of the yarn, degradation of the quality of the yarn due to fatigue caused by twisting, and decrease in the life of the periphery of the discs would be resulted.
These difficulties are caused by a large frictional resistance of the disc causing balooning of the yarn. More particularly, when a yarn Y moves from one intersection of the peripheries of the discs to the other, it engages obliquelly against discs 3 and 4. At disc 4, the angle .alpha..sub.1 between the direction of advance of the yarn Y and the direction of rotation of the disc is smaller than 90.degree. so that the frictional force F caused by the rotation of disc 4 imparts a component F.sub.1 = F cos .alpha..sub.1 to the yarn in the direction of advance thereof. However, at the other disc 3 since angle .alpha..sub.2 is larger than 90.degree., the yarn will be imparted with a component F.sub.2 which is equal to component F.sub.1 but acting in the opposite direction. For this reason, with such apparatus, any yarn feeding or advancing action is not produced but the frictional resistance increases whereby the ratio between twisting tension and untwisting tension increases. Consequently, it is difficult to obtain an adequate twisting tension thus making unstable the twisting action of the yarn.
Although it has been proposed to cause a yarn to obliquely engage a disc so as to create a yarn feeding force, such proposal is not satisfactory in that it is impossible to obtain sufficient frictional force thus resulting in an insufficient number of the twists and that it is necessary to provide suitable guides on the front and rear sides of the discs so as to cause the yarn to obliquely engage the disc. However, such guides manifest resistance to the twisting action thus resulting in an insufficient number of twists.
To obviate these difficulties it has also been proposed to substitute discs 3 and 4 which are adjacent each other by discs 3a and 4a as shown in FIG. 1A. Discs 3a and 4a haver a little larger thickness than other discs 1, 2, 5 and 6 and their peripheries are generally conical and curved outwardly. Although this improvement can substantially eliminate the difficulties of the apparatus shown in FIG. 1, it was found that the path of travel of the yarn between discs 3a and 4a varies frequently between the states shown in FIGS. 1B and 1C. When twisting a nylon yarn the number of twists is not sufficient under the state shown in FIG. 1C whereas when twisting a polyester yarn the tension of the yarn varies greatly under the state shown in FIG. 1B. Under the state shown in FIG. 1C when the speed of the yarn exceeds 300 m/min. stick slip occurs on disc 4a resulting in insufficient twisting back.